Twelve years ago, after extensive auditioning at several dealers, I sold my NHT 3.3s and bought a pair of Revel Ultima Studios. It was a very exciting change -- until I got them home, and they sounded an awful lot like the old speakers.
Then, I had ASC draw up an acoustic plan for my room, and I installed the recommended traps and panels. When I was done, it was great! THAT was the biggest improvement I've made to my system, in 45 yrs of audiophilia. I wound up spending half as much again on acoustics as I did on the (rather costly) speakers, but it was worth it. From organ music to harpsichord, it all sounded better, clearer, more dynamic, less irritating, less electronic, more natural.
The second-biggest change -- but still VERY significant -- was trading in my c-j MF-2300 amp for a 14B-SST. Clearer sound throughout the spectrum, and FAR better bass. I remember dancing to Bob Marley when we first plugged it in. Wow!